A known image forming apparatus is configured to form an image on a recording medium, e.g., a sheet of paper, while conveying the sheet on an endless belt. The image forming apparatus may include a color laser printer of a direct transfer tandem type. In the image forming apparatus, a sheet to be conveyed by the endless belt may be subject to a phenomenon where a center portion of the sheet in a direction perpendicular to a sheet conveying direction (hereinafter referred to as a width direction) is conveyed in advance of side portions of the sheet in the width direction. Hereinafter, the phenomenon is referred to as “center portion advance conveyance.”
In a case where a plurality of sheets are left stacked in conditions of high temperature and high humidity, if upper stacked sheets are excluded, only peripheral edges of the remaining sheets may be exposed to the hot and humid surroundings. Thus, the sheets may remain dry in the center portion while being damp in both side portions, in the width direction, where water is absorbed from the air. Such sheets may be stiff in the center portion and soft in both side portions, and thus may suffer some warping at both side portions.
During conveying of sheets, if a center portion of a sheet is conveyed in advance of side portions of the sheet before the sheet is conveyed to the endless belt, a center portion of a leading end of the sheet contacts the endless belt in advance of side portions of the leading end of the sheet in the width direction. Thus, the sheet may suffer forces acting from both sides of the sheet to the center portion thereof in a direction from the leading end to a trailing end of the sheet.
If a sheet has a stiff center portion and soft side portions, forces acting from both side portions to the center portion may react upon the elasticity in the center portion of the sheet, and thus the sheet may crease. If a toner image is transferred to a sheet having a crease in the center portion, the toner image is improperly transferred in a creased portion of the sheet. In addition, a sheet having a crease may not be properly conveyed. For these reasons, it has been proposed to prevent the center portion advance conveyance by applying resistance to a center portion of a leading end of a sheet in the width direction greater than to side portions of the leading end of the sheet in the width direction when the sheet is conveyed to the endless belt.
However, when resistance is applied to the center portion of the leading end of the sheet in the width direction, a phenomenon may occur that the center portion of the leading end of the sheet is conveyed later than the side portions of the leading end of the sheet. This phenomenon may occur irrespective of surrounding environment such as temperature and humidity, and a type of a sheet. Thus, it is necessary to apply resistance to the center of a sheet in conditions of high temperature and high humidity where a crease occurs in the sheet due to the center portion advance conveyance. However, it is not necessary to apply resistance to the center of a sheet in normal conditions where a crease is less likely to occur in the sheet even if the center portion advance conveyance occurs. Thus, providing for a switching member to switch whether resistance is applied or not is proposed.
However, as the switching member is manually operated, additional work is required and the structure of the device is complicated. If a user fails to operate the switching member, an unacceptable image may be formed. Moreover, whether a sheet creases, in other words, whether resistance is applied to a center portion of a sheet, depends on not only temperature and humidity but also stiffness of the sheet.